Makes sure a given element is in the visible area of its scroll container;

ensure-visibility.js

function ensureVisibility(e, c) {
	let e0 = e.offsetTop
	let eD = e.clientHeight
	let e1 = e0 + eD
	
	let c0 = c.scrollTop
	let cD = c.clientHeight
	let c1 = c0 + cD
	
	let visible =  e0 > c0 && e1 < c1
	
	if (!visible) {
		if (e0 <= c0) {
			// Stick at container top
			c.scrollTop = e0
		} else {
			// Stick at container bottom
			c.scrollTop = e1 - cD
		}
	}
}